Atmospheric Composition                      

Our lives depend on a delicate balance
of  invisible gases that are minor components
of  the Earth's atmosphere.  Carl Sagan

The radiative properties of Earth's atmosphere are strongly influenced by its composition.  As we have discussed in our review of the satellite image interpretation tutorials, the radiance recieved at a satellite is a result of electromagnetic radiation that undergoes several processes which are wavelength dependent:

The composition of the atmosphere influences both the incoming solar radiation and the outgoing terrestrial radiation.  The following table illustrates the composition of the dry atmosphere.  The water vapor content of the atmosphere is highly variable, ranging from 1-4% of the total gaseous composition.  In fact, the presence of relatively large amounts of water vapor in the atmosphere is very important in atmospheric dynamics and plays a key role in the redistribution of energy.
Composition of Dry Air at Ground Level in Remote Continental Areas
Constituent Formula Concentrations
Nitrogen N2 78.1%
Oxygen O2 20.9%
Argon Ar 0.93%
Carbon dioxide CO2 0.035%
Neon Ne 0.0018%
Helium He 0.00050%
Methane CH4 0.00017%
Krypton Kr 0.00011%
Hydrogen H2 0.00005%
Ozone O3 0.000001-0.000004%
(10-40 parts per billion by volume)

Adapted from Atmospheric Change, T.E. Graedel and P.J. Crutzen

However, disregarding water vapor, we see that 99.9% of the earths atmosphere is composed of nitrogen, oxygen and the chemically inert noble gases.  Carbon dioxide, which is chemically unreactive in the troposphere, along with methane, ozone, and water vapor are essential for making our planet inhabitable.  These molecular species have the ability to absorb longwave radiation (emitted from the surface) through a number of vibrational and rotational energy transitions.  

Most of the really high energy solar radiation, with wavelengths less than 100nm gets absorbed above 100km by N2, O2, N, and O and their ions.  At wavelengths longer than 100nm, the radiation is not absorbed by N2, N or O, however, molecular oxygen, O2, is still a strong absorber at these wavelengths. In the upper atmosphere, these high energy photons are absorbed by oxygen molecules, producing atomic oxygen. Almost all of the photons <210nm are absorbed above 50km.  In the stratosphere, there is enough molecular oxygen present (as the air density increases) that some of the oxygen atoms will combine with oxygen molecules producing ozone. At wavelenths in the range of 210-310nm, O3 is a major absorber.  This ozone absorption provides energy that heats the stratosphere.  These absorbing gases are also essential as a filter, screening that portion of the incoming solar radiation which is most harmful to biological systems.  At wavelengths longer than 310nm, there is very little attenuation of radiation by absorption.

Consider the following temperature profile which we refer to as the Standard Atmosphere, an idealized average condition.  We can interpret this structure in temperature in terms of the atmospheric composition and its wavelength dependent interaction with radiation.

Temperature Structure (missing figure- see overhead in class).

In the thermosphere ultraviolet radiation results in dissociation, ionization and heating.  The mesopause minimum results from the lack of absorption in this region. The stratospheric peak in temperature is a result of the slightly longer wavelength ultraviolet absorption by ozone. The minimum T at the tropopause is due to the insignificant amount of solar absorption in this region. Incident radiation at these longer wavelengths (>310nm) is transmitted, or scattered and reflected The portion that is not reflected reaches the surface as direct + diffuse radiation.  This incident radiation effectively warms our atmosphere from below, therefore, we see that throughout the troposphere the temperature decreases with height (ie., away from the heat source).

Definitions of Radiative Processes

Definition adapted  from the Glossary of Meteorology, R.E. Huschke, ed., American Meteorological Society, 4th edition, 1986.

  1. Absorption:  The process by which incident radiant energy is retained by a substance.  Absorption results in the irreversible conversion of radiation into some other form of energy within and according to the nature of the absorbing medium.  The absorbing medium itself may emit radiaton, but only after an energy conversion has occurred.

    It is important to keep in mind that a medium which absorbs radiation may also reflect, refract, diffract or scatter radiation, however these processes do not retain or transform the energy.  

    An absorption line is a minute range of wavelenght (or frequency) in the electromagnetic spectrum within which radiant energy is absorbed by the medium through which it is passing.  Each line is associated with a particular mode of vibration or rotation induced in an absorbing molecule by the incident radiation.  An absorption band of a polyatomic gas is actually made up of closely spaced groups of absorption lines.

  2. Transmission: The process by which radiation is propogated through a medium.  Measured as transmittance, the ratio of the transmitted radiaition to the toal radiation incident upon the medium.
  3. Upper figure credit: UCSB Remote Sensing Core Curriculum

    Lower figure credit: J. Key, Boston Univ.


                             <Review several figures illustrating the  absorption by variou  atmospheric components as a function of wavelength and the corresponding reduction in solar irradiance from the top of the atmosphere to the surface>


  4. Reflection: The process whereby a surface of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached.  

    For true reflection, there must be a real discontinuity of the index of refraction or at least it must change over an interfacial layer of thickness small compared to the wavelength of the radiation.  If the change of refractive index is gradual (for example, in a stratified medium) radiation may be returned by the process of refraction, not reflection.  

    The process of reflection is not affected by wavelength except as the relative scale of the irregularities of the surface change with wavelength.  When the scale of irregularities of the reflecting surface is small compared to the wavelenght, regular, or specular reflection results, which means 1) the reflected ray lies in the plane defined by the incident ray and the normal to the surface at the point of incidence and 2) the angle of reflection equals the angle of incidence (both measured from the normal to the surface).   When the surface irregularities are large compared to the wavelength, diffuse reflection occurs;  surfaces with intermediate reflection properties are called semi-matte or semi-gloss.  

    The fraction of incident radiation does depend on wavelength because of the selective nature of the absorptivity and transmissivity.  The idealized white body is a total reflector; an ideal black body reflects none of the incident radiation.  

  5. Scattering: The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions.  In scattering, no energy transformation results, only a change in the spatial distribution of the radiation.  Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere.  

    Scattering is a function of the ratio of the particle diameter of the material doing the scattering to the wavelength of the incident radiation.  

    When the scattering material has a diameter < 1/10th the wavelength, Rayleigh scattering results. Rayleigh (an English mathematician and physicist, 1842-1919) found that this scattering was proportional to the inverse of the wavelength raised to the fourth power.  Based on this relationship, air molecules (N2 and O2) are just the right size to very effectively scatter the shorter wavelengths (blue light) of incident solar radiation.  This results in a blue sky.

     Mie scattering (Gustav Mie, 1868-1957, a German physicist) becomes important when the diameter of the scattering material is >1/10th the wavelength, so for example, when there are larger particles present.  

    The scattering occurs in all directions, so the sky appears as a dark, saturated blue dome, whitening toward the horizon as the pathlength increases, and there are more particles present.  If it weren't for scattering by air molecules, the sky would look black.  Since this scattered radiation is dispersed in all directions, some is sent back to space, the rest reaches the surface as diffuse (or sky) radiation, rich in blue.  

Taking into consideration all the processes mentioned above, the solar irradiance at the surface is composed of all the radiation which has not been absorbed or reflected, and we can think of this as having two parts:

Direct beam insolation varies as a function of solar altitude, which determines pathlength and the surface area over which the radiation is spread.  It is influenced by season, time of day, latitude.  Diffuse radiation is a function of cloud cover, aerosol distribution, etc.

What is the effect of solar energy on the earth surface/lower atmosphere system?  Some of the energy goes into internal energy of the system (it warms things up).  Some of this energy goes into doing work on the system, increasing the organization (forming and maintaining thermal gradients).  Some goes into forming potential energy gradients (throught the evaporation and transportation of water vapor through the system).

We know that the satellite measured radiance is a function of both wavelength and temperature, so in our next lecutre we will go on to consider variations in atmospheric temperature.  We need to be able to understand the horizontal and vertical temperature structure to interpret and to apply  and to understand the limitations of satellite radiance observations.

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Atmospheric Thermodynamics